This invention relates generally to apparatus for making material such as paper and the like. More particularly, this invention relates to a method and apparatus for measuring the paper basis weight or other parameters to make either cross directional or machine directional adjustments in the paper thickness.
In a typical process for making paper and related sheet products, a headbox is provided for receiving and containing a mixture of cellulose fiber and water. The contents are discharged through a slice lip on the headbox onto a moving wire to form a web of sheet material. In the process of making paper from this material web, maintaining an optimal basis weight is desired. Basis weight is defined as the total weight of the material per unit area, and is usually stated in grams per square meter. The basis weight of the material is a function of the cellulose flow rate into the headbox, the gap of the slice lip through which the mixture passes onto the wire and the wire speed. A stock valve between the headbox and a cellulose source controls the cellulose flow rate, while slice lip actuators control the slice lip gap.
Basis weight has traditionally been determined with nuclear (radioactive isotope) -based sensors that measure the attenuation of incident radiation, X-rays, beta rays, etc. through the paper web. The nuclear sensor, because of its cost and safety concerns, is usually a single sensor mounted for scanning back and forth across the width of the web (cross direction), rather than a number of stationary sensors aligned in the cross direction. The information gathered in each scan is processed and saved in a computer memory as a series of data boxes, each box representing a defined cross web position. For example, if the width of the paper web is 400 inches and each data box is one inch wide, then data is gathered for the 400 data boxes with each having a known cross web position.
The mean value of the samples of information gathered in a scan across the width of the paper web as it is being pulled through the machine is the scan average. The scan average is used for machine direction control of the basis weight through control of the stock valve, which modulates the flow of cellulose fibers into the headbox. The information gathered for each data box is used for cross direction (profile) control of the basis weight through control of the slice lip actuators, which adjust the slice lip gap across the headbox. Further background information on the structure and operation of paper making machinery may be found in U.S. patent application Ser. No. 629,093 filed Dec. 17, 1990, which is hereby incorporated by reference.
Although accurate, nuclear sensors have a frequency response limit of about 200 Hz. That is, such sensors require at least 0.05 seconds to accurately measure a change in a parameter such as the paper basis weight. This presents a drawback because of the need for fast scan speeds to generate an optimum number of scan averages for machine direction control of the cellulose flow via the stock valve. The faster the scan speed, the more frequent and more accurate the control. However, the scan speed of a nuclear sensor is limited by the frequency response of the sensor as well as white noise that it generates. The white noise can be as great as 10% of a desired measurement.
For profile control, the rate of change across the width of the paper web can far exceed the frequency response of the nuclear sensor. And as in machine direction control, there exists a large band of noise in measuring the profile which severely limits the ability to accurately control the slice lip actuators to provide a desired basis weight.
Sensors that may be scanned at a faster rate, such as optical sensors, are available, but none offer the long term stability of a nuclear sensor. U.S. Pat. No. 4,289,964, for example, discloses apparatus in which a stationary beta gauge is augmented by an optical scanner. The output signal of the beta gauge in one specific alignment of the paper web is compared against the output of the optical scanner in an effort to correct for signal drift that occurs in the output of the scanner. While this method of comparing the output of the two sensors offers some improvement over a nuclear-based scanner, the output is still inaccurate. For most of the web width, the web portion scanned by the optical scanner is different from the web portion measured by the beta gauge. The portions are the same only when the optical scanner is aligned momentarily with the stationary beta gauge as the optical scanner sweeps across the sheet material.